Mono Instance) mode does not use launching user
Using Notepad++ in default (Mono Instance) mode does not use launching user
When you use multiple users on one system, Mono Instance will launch new edits from user2 in the NP++ session of user1
This can cause autorisation issues as user1 can sometimes not access the file
NP++ should take the user in account and launch a second instance if launched from another user
That is a reasonable suggestion.
If you want to increase the chances of the developer noticing this suggestion and possibly implementing it in a future release, you can follow the instructions in this FAQ to add a feature request in the official issues-page.